The Oil & Gas upstream production industry drills wells of ever increasing depth and complexity to find and produce raw hydrocarbons. The industry routinely uses steel pipe (Oil Country Tubular Goods) to protect the borehole (casing) and to control the fluids produced therein (tubing). Casing and tubing are made and transported in relatively short lengths and installed in the borehole one length at a time, each length being connected to the next. As the search for oil and gas has driven companies to drill deeper and more difficult wells, the demands on the casing and tubing have grown proportionately greater in terms of both tensile and pressure forces. The developing technology of deviated and horizontal wells have exacerbated this trend, adding to the casing and tubing requirements a further consideration of increasing torsional loads.
Two general classes of connectors exist within this field. The most common is the threaded and coupled connector, wherein two pin, or male threads, which are machined on the ends of two long joints of pipe, are joined by two box, or female threads, machined on a relatively short member, a coupling, with a larger outside diameter than the pipe, and approximately the same inside diameter. The other class is the integral connector, wherein the pin member is threaded onto one end of a full-length joint of pipe and the box member is threaded into the second full-length joint. The two joints can then be directly joined without the need for an intermediate coupling member. The ends of the pipe body may be processed further to facilitate the threading of the connection.
The prior art demonstrates the use of a variety of different thread, shoulder and seal configurations for oil country tubular goods. One type of shoulder and seal combination used in the prior art is the so-called center shoulder seal configuration. As used herein, a “center shoulder seal” is understood to mean a section of a connection disposed between at least two threaded portions, which section exhibits directly contacting surfaces between the pin and box members, or between the pin and box members and a third component such as an annular sealing sleeve, so that the section provides a seal for preventing the passage of liquid or gas through (across the threads of) the assembled connection.
U.S. Pat. Nos. 5,415,442 and 5,462,315 disclose a center shoulder configuration, represented in FIG. 1, in which a total of five metal-to-metal seals are created during assembly (make-up) of the connection: two metal-to-metal seals at the exterior beveled edges of the locked double shoulder seal 400 where sealing surfaces 418 and 438 are in contact and where sealing surfaces 422 and 442 are in contact; and, three zero clearance surfaces at the interior of the locked double shoulder seal 400, one 410 at the wall surface where sealing cylindrical surfaces contact, another zero clearance surface where the annular face surface of the pin shoulder 420 contacts the box undercut surface 448, and another zero clearance surface where the face surface of box shoulder 440 contacts pin undercut surface 428. Thus, center-shoulder seal 400 forms a very close fitting metal-to-metal seal having mating metal-to-metal sealing surfaces which allow a build-up of stored energy within the seal upon power tightening of the assembled connection, such that upon the application of various loads on the pipe and also the connection, the seal will continue to perform and maintain sealing engagement. Notably, all of the metal-to-metal seals are formed within the axial space 450 between the sets of contacting annular faces of the center shoulder, and the metal-to-metal seals at the exterior beveled edges of shoulder seal 400, each of which exhibits radial forces, are each located immediately adjacent a respective one of the sets of contacting annular faces.
U.S. Pat. Nos. 5,765,836 and 6,041,487 disclose a center shoulder configuration, represented in FIG. 2, in which a total of at least two and up to seven metal-to-metal seals are established between the pin and box members. Three of the seven seals are frustum seals. A first outer frustum seal 120 is formed by the engagement of pin and box frustoconical surfaces at the radially outer side of the center shoulder and a second inner frustum seal 122 is formed between pin and box frustoconical surfaces at the radially inner side of the center shoulder. The third frustum seal is a center frustum seal 124 formed between intermediated pin and box center frustoconical surfaces. Two annular shoulder seals are formed. A first inner annular shoulder seal 126 a second outer annular shoulder seal 128, each by engaged annular surfaces of the pin and box members. Two cylindrical seals are also formed. A first outer cylindrical seal 130 is formed by the engagement of pin and box outer seal cylindrical surfaces at the radially outer side and a second inner cylindrical seal 132 is formed by the engagement of pin and box seal cylindrical surfaces at the radially inner side. Notably, this arrangement also places each of the metal-to-metal seals that are exposed to significant radial and hoop forces (i.e., contacting seals 122 and 120) immediately adjacent a respective one of the sets of contacting annular faces (i.e., at annular contacting seals 126 and 128) and likewise within the axial space 150 between the two annular shoulder seals.
In the center shoulder configurations of the foregoing patents, axial loading within the shoulder connections tends to produce the highest stresses proximate the axial locations of annular shoulder contact (e.g., at annular shoulder seals 126 and 128 in FIG. 2, and similarly at the annular shoulder seals formed in the arrangement of FIG. 1). Thus, highest stresses and material yield or deformation tends to occur within or near the axial region 450, 150 that, in each case, is defined between the annular shoulders of the connection.
What is needed is a center shoulder connection that provides effective sealing while avoiding potential issues of radial sealing forces in the region of the high stress and likely yield area of the center shoulder.